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Abstract

Two catastrophic and destructive landslides happened as a consequence of a major earthquake on the midnight of June 20, 1990. Their morphology and dynamic analysis are scope of the present study.

The study area is placed in a mountainous (Alborz Mountain Range) and highly seismic active region of Iran. Recent active faults in the different parts of the range are subjected to new investigation on the seismicity of the Alborz. It will describe later in the seismicity chapter that two sets of old reactivated thrust faults (eg. Moinfar and Naderzadeh, 1990) or recent strike slip faults (eg. Berberian et al. 1992) could have been responsible for most of the major earthquake. Most of the thrusts have a component of strike slip. There are a number of younger faults in the Alborz which are predominantly of strike-slip type.

As a major consequence several landslides were triggered by the earthquake; most of them were not catastrophic. The term landslide covers most down slope movements of rock and solid debris that have become separated from the underlying part of the slope by a shear zone or slip surface. Number of landslides has counted from 76(Haeri 1996) to 140 (Shoaie and Sassa 1993). A few of them were disastrous while killed many people and destroyed villages, roads and farms.

Among the catastrophic landslides in the area two of them are well known because of their large amounts of damage (Rudbar landslide) and loss of life (Fatalak Landslide).

The Rudbar and Fatalak landslides are located in an active seismic area (Figure 1 2) that subjected to investigate in the present literature are among the catastrophic mass movements but not in the same manner.

The landslide in Rudbar is a classic debris slide-debris flow that happened in mostly disturbed Jurassic shale and sandstones, covered by a thin layer of Post glacial (Holocene) sediments. This long landslide with nearly 2.4 km length demolished productive gardens of olive which make the most important income of the local people, destroyed thousands of trees in the area. An estimated area of 80 hectares is not producible any more. According to the witness interview and published reports the landslide took about 10 days to move from its main escarpment to toe (Appendix 9-1). Total volume of the removed material is estimated about 10 million cubic meters.

Unlikely the Fatalak landslide with 2.5 million cubic meter volume and 40 hectare area is classified as a very rapid debris flow as its materials contains of grain size from boulder to clay. The landslide occurred mostly in Quaternary alluviums covered Cretaceous volcanic rocks. The mass buried many houses of four villages and killed as many as 170 residents.

The landslide occurred as a reactivation of a bigger landslide.

As the study shows the area is divided to several parts, each part is contributing as a single event in the land slide. It is necessary to mention that the numbers pointing the position of morphologic patterns and event are not same as predefined picture in section (5-2).

The landslide started to crack down at the moment of the earthquake. The slide started to fail in its middle part as a consequence of reactivating an existing fault (dashed red line in the figure). The fault has been reported by Nazari and Salamati (1998) in the geological map of Rudbar. The fault causes a small slope failure at the northern side of the body (blue curve in the figure).

Strong shaking at a short time (horizontal ground acceleration was about 0.6g at the nearest station at 40 seconds) severely affected water table that could be at 10 meter depth.

The first stage took place after the ground motion. A distinct fracture is appeared in the middle of the landslide after the event. This activity severely damaged to high water table and decreased the available shear strength. As a result two simultaneous escarpments occurred in 40 seconds after the main shock.

For ten days after the event the area was exposed to a debris flow that traveled with the velocity of 35m per day. Final passage of the landslide was in the nearest point to the residents of Rudbar city.

The Fatalak landslide happened in a soil with different strength parameters, soil characteristics such as grain size and plasticity index compare with the Rudbar landslide. Also the slop angle was bigger than the Rudbar landslide.

Based on dynamic response analysis the slope failed at about 80 second after the earthquake after the major earthquake.

Based on numerical simulation done by F. De Blasio the slide body had a very fast velocity after the start point. The velocity was reached to 40m/sec.

Morphologically the slide contains of three phases:

The first stage is happened in the upper part of the slide but not in the highest escarpment. Retrogressively the slide created some steps and escarpments that become bigger upslope.

Based on witnesses and GIS result the Fatalak village that devastated by the slide material was located in front of an old but short escarpment. The village also had some water springs. It shows the slide material in the area was exposed to flux of water from the higher elevation. This flux could be one of the main causes for creating the slip surface.

The slide passed a transition area to convert its erosion pattern to accumulation. This zone is about 200 meter long and its depth is a variation from 1 to 4 meters.

After the second part, the slide body reached to the main drainage of the valley in the western part of the slide body its velocity decreased and stopped in the river and some places in the opposite flank. The eastern flow of material that filled the drainage moved down stream to the lower point elevation in the valley. These materials covered the western accumulation in some places and also bury some residents in the downslope. The eastern flow had a longer passage because of topographical situation and stopped after some times. Despite reports about existence of two separate escarpments no sign of that found during the investigation. However some differences in the terrain of the slide show that the slide contains of two debris flows in its eastern and western part. The eastern part traveled a longer distance than the western part.